1. Field of the Invention
The present invention relates to a substrate holding apparatus or mask stage base in an exposure apparatus such as an X-ray exposure apparatus in which synchrotron radiation light is utilized as an exposure light.
2. Related Background Art
In an X-ray exposure apparatus, an optical axis thereof is substantially parallel to a reference plane or a horizontal, differently from an ordinary exposure apparatus, so that it is required that a mask or substrate and a wafer for exposure be vertically held and that this relationship be maintained.
FIG. 1 shows a mask hand 4 for transporting an X-ray mask 1 and a mask stage 8 to be used as a base holding apparatus, which are used in a prior art X-ray exposure apparatus. In the X-ray mask 1 of FIG. 1, a mask membrane 2 having a mask pattern thereon is fixed to a mask frame 3 containing magnetic material. Mask hand 4 has a hand body 5 and two fingers 6a and 6b pivotally mounted to both sides of the hand body 5. The mask 1 is held by closing the fingers 6a and 6b, and the mask frame 3 is attracted to a magnetic unit 7 fixed to a central portion of the hand body 5. Thus, the X-ray mask 1 is held vertically and is handed to a mask stage 8.
In the mask stage 8, a magnetic unit (not shown) is embedded for engaging or disengaging the X-ray mask 1 with or from a stage surface or substrate holding surface. On the stage surface, a V-block 9 is mounted for indicating a location where the X-ray mask 1 is positioned. The magnetic unit embedded in the stage surface includes a permanent magnet and an electromagnet for generating a magnetic field for offsetting a magnetic field of the permanent magnet when energized. The holding of the mask 1 is performed by the permanent magnet, while the disengagement of the mask 1 is caused by energizing the electromagnet. The prior art technique, however, has the following disadvantages.
In the substrate holding apparatus for holding the substrate for an X-ray exposure, there is a possibility of dropping the substrate since the substrate is vertically held. But, no means for preventing the dropping of the substrate is provided because such means would obstruct the movement of the mask hand 4 and a handling operation of the substrate performed when the substrate is transferred on the mask stage 8, and for other reasons. Therefore, in the case of a mask stage with a magnetic unit, the substrate falls downward by gravitational forces if the electromagnet is erroneously energized when the mask 1 is held. This may result in not only damaging the X-ray mask 1, but also damaging devices located under the mask stage 8. Thus, safety of the substrate holding apparatus cannot be ensured and its reliability is lowered. Further, economic burdens become large because the X-ray mask 1 is expensive.
Another prior art mask stage base is shown in FIG. 2. On a mask substrate 11, a highly integrated mask pattern 12 is formed, and the mask substrate 11 is adhered to a supporting frame 13 formed with heat resisting glass, metal or the like, by adhesive. The alignment or positioning of the mask at the time of exposure is conducted by bringing side walls of the supporting frame 13 into contact with each of three positioning pins 14, 15 and 16 studded on a mask stage base 17 on which the X-ray mask structure 11-13 is mounted. After the alignment of the mask structure, the mask structure is held by a vacuum holding mechanism (not shown) provided on the mask stage base 17.
The above-mentioned mask stage base 17 holds the X-ray mask structure horizontally because X-rays for exposure are applied from above. In contrast, in a case when a synchrotron radiation light is used as an X-ray source, it is required that the X-ray mask structure be vertically held by a vertically standing mask stage base due to the structure of a synchrotron radiation apparatus.
In order to position the X-ray mask structure by the prior art positioning mechanism in the above-mentioned condition, the mask structure must be pushed against one positioning pin while pushing the mask against the remaining two pins. But, in order to perform that operation by the transporting equipment, a sophisticated mechanism is needed. Further, while there exists another method for positioning the mask structure by positioning the mask stage base with a six-axis stage without accurately positioning the X-ray mask structure directly, a structure therefor also becomes complicated.
Further, there is a problem that when trouble occurs in a vacuum holding mechanism in the mask stage base of FIG. 2 (for example, a leak occurs in a vacuum line in a vacuum holding system, or an electric source fails in a magnetic attracting system), the X-ray mask structure being held by vacuum ceases to be attracted and falls downward due to its own weight.